An usual contract at Jefferson National Accelerator Facility (JLab) in Newport News, Va., called for development of two mixes: one for a primary wall and roof member layer, prepared with up 90 lb./cu. ft. of boron carbide, a compound widely used in radiation shielding and processed in 4- to 125-micrometer gradations; the other for weight-sensitive wall sections and combining expanded shale and recycled polypropylene aggregate. The boron carbide (equal to normal weight) and lightweight concretes—coupled with lead, aluminum and boron carbide composite layers—form a 25- x 20- x 14-ft. shield house where JLab physicists will gauge subatomic particle behavior following electron-nucleus collisions.

Over a three-year testing and approval phase, Branscome and Essroc staff refined a self-consolidating boron carbide mix with radiation-containment properties exceeding JLab physicists’ targets. Wall and roof member cross section streamlining from 9 in. to 6 in. thickness equated to about $250,000 in savings, primarily due to a reduction in volume of $10/lb. boron carbide aggregate. The concrete schedule has seen 10 yd. of the boron carbide mix, which Branscome crews have prepared on site in 2.2 cu. ft. batches, greatly minimizing the amount of wasted material accompanying truck delivery. Finished wall and roof member costs place the boron carbide mixes at $30,000/yd.

The boron carbide concrete blocks one grade of subatomic particles. The considerably less expensive lightweight aggregate concrete is absorbs other particles, principally because of an elevated hydrogen environment attributable to the plastic and water-retaining shale aggregate. Branscome has produced 275 yd. of the lightweight aggregate mixes at its Williamsburg headquarters plant, typically dispatching 8 yd. loads for 24- to 35-in. thick shield house wall pours.

Branscome and Essroc performed the mix development and concrete placement for Jefferson Science Associates, which runs JLab under contract for the U.S. Department of Energy. The boron-rich concrete technology, “Thermal neutron shield and method of manufacture,” has been granted patent #8,450,707 by the U.S. Patent and Trademark Office; companion technologies, “Lightweight Con¬crete with Enhanced Neutron Shielding” and “Cost Effective Boron Shield¬ing Panels” are patent-pending. All three technolo¬gies are available for licensing through Jefferson Science Associates. The JLab shield house is the subject of the February Concrete Products cover story and can be viewed here. Additional background on the project is posted at www.jlab.org.

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